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United States Patent |
5,057,674
|
Smith-Johannsen
|
October 15, 1991
|
Self limiting electric heating element and method for making such an
element
Abstract
This invention relates to a self-limiting electrical heating element
comprising resistance components having a positive temperature coefficient
and a zero temperature coefficient, arranged in a layered structure with
two electrodes placed diagonally within or in contact with two ZTC layers
separated by a PTC layer.
Inventors:
|
Smith-Johannsen; Robert (Borgheim, NO)
|
Assignee:
|
Smith-Johannsen Enterprises (Incline Village, NV)
|
Appl. No.:
|
555424 |
Filed:
|
August 8, 1990 |
PCT Filed:
|
January 30, 1989
|
PCT NO:
|
PCT/NO89/00011
|
371 Date:
|
August 8, 1990
|
102(e) Date:
|
August 8, 1990
|
PCT PUB.NO.:
|
WO89/07381 |
PCT PUB. Date:
|
August 10, 1989 |
Foreign Application Priority Data
Current U.S. Class: |
219/553; 219/504; 219/505; 338/22R; 338/25 |
Intern'l Class: |
H05B 003/10 |
Field of Search: |
219/504,505,510,511,540,553,528,548,494
338/22 R,25,105
|
References Cited
U.S. Patent Documents
4017715 | Apr., 1977 | Whitney et al. | 219/553.
|
4177376 | Dec., 1979 | Horsma et al. | 219/553.
|
4330703 | May., 1982 | Horsma et al. | 219/553.
|
4334148 | Jun., 1982 | Kampe | 219/553.
|
4429216 | Jan., 1984 | Brigham | 219/528.
|
4543474 | Sep., 1985 | Horsma et al. | 219/553.
|
4654511 | Mar., 1987 | Horsma et al. | 219/548.
|
4689475 | Aug., 1987 | Kleiner et al. | 219/553.
|
4800253 | Jan., 1989 | Kleiner et al. | 219/553.
|
4801784 | Jan., 1989 | Jensen et al. | 219/548.
|
Primary Examiner: Reynolds; Bruce A.
Assistant Examiner: Switzer; Michael D.
Attorney, Agent or Firm: Pennie & Edmonds
Claims
I claim:
1. Self limiting electric heating element including two outer
semiconductive layers (1, 2; 11, 12) having zero temperature coefficient
(ZTC) separated from one another by a continuous positive temperature
coefficient (PTC) layer (3; 13) and energized by two parallel electrodes
(4, 5; 14, 15), one of which is in contact with one end of one of the ZTC
layers and the other parallel electrode is in contact with the other ZTC
layer at its end furthest removed from said one end, characterized in that
the resistances of the PTC and ZTC components have the following
characteristics: at room temperature:
RPTC1<<RZTC<<RPTC2,
and at control temperature:
RPTC1=RZTC,
where RPTC1 is the electrical resistance measured across the PTC layer,
where RZTC is the resistance of the two ZTC layers connected in parallel,
each having a resistance of 2.multidot.RZTC,
where RPTC2 is the resistance measured between the electrodes (4, 5; 14,
15) along the PTC layer,
so that at the control temperature the heat generated per time and unit
area, i.e. the watt density, of the PTC layer and the watt density of the
two parallel ZTC layers are essentially equal.
2. Heating element according to claim 1, characterized in that at control
temperature in an element having sides d, 1, where l=d, a thickness t(PTC)
of the PTC layer and a combined thickness t(ZTC) of the two ZTC layers,
the ratio of resistivity of the PTC layer to that of the ZTC layers are:
d.sup.2 /(t(PTC).multidot.t(ZTC)).
3. Method of making a heating element according to claim 1 or 2,
characterized in that the PTC layer (3; 13) is made from 20 to 50 parts of
a large particle carbon black such as Elftex.TM. carbon in 100 parts of a
thermoplastic resin such as PE or EVA and that the compound is made into a
0.025 to 0.2 cm thick film at a required resistivity.
4. Method of making a heating element according to claim 1 or 2,
characterized in that the ZTC layers (1, 2; 11, 12) are made from a glass
scrim impregnated with an aqueous dispersion of highly conductive carbon
black, e.g. Ketchen Black.TM..
5. Method according to claim 4, characterized in that the ZTC carbon black
is run through a fluid energy machine along with 5 to 30% by weight of
some 40% aqueous colloidal silica, e.g. DuPont Ludox HS-40 .TM. and that
this composition is dispersed in water along with polyethyleneimine.
6. Method according to claim 5, characterized in that the ZTC carbon black
mixture is modified with a binder consisting of an acrylic latex, clay and
aqeous colloidal silica and also polythyleneimine, the binder being in a
proportion to produce the desired resistance level on the coated scrim.
Description
FIELD OF THE INVENTION
This invention relates to a self limiting electric heating element
including two outer semiconductive layers having zero temperature
coefficient ("ZTC") separated from one another by a continuous positive
temperature coefficient ("PTC") layer and energized by two parallel
electrodes, one of which is in contact with one end of one of the ZTC
layers and the other parallel electrode is in contact with the other ZTC
layer at its end furthest removed.
BACKGROUND OF THE INVENTION
There are known several types of self limiting electrical heating elements
having geometrical configurations similar to those preferred in the
invention. Such heating elements are known from, e.g., German patent No.
2,543,314 and the corresponding U.S. Pat. Nos. 4,177,376, 4,330,703,
4,543,474, and 4,654,511.
The heating elements described in said German patent DE-C2-2 543 314 relate
in particular to heat recoverable articles. These articles are mostly used
for sealing purposes such as covers for electrical components and cable
joints. The heat recoverable article is arranged to be placed around the
component or joint to be sealed, whereupon the article is connected to a
power supply. The compositions and combinations of layers constituting the
article are chosen such that the article is heated to a defined
temperature at which the article shrinks and seals the electrical
components or cable joint.
Similar heating elements are also described in U.S. Pat. No. 4,017,715 and
EP-A1-0 237 228. The requirements of the elements described in U.S. Pat.
No. 4,017,715 are that at room temperature the resistance in the ZTC layer
is greater than the resistance exerted in the PTC layer.
From U.S. Pat. No. 4,689,475 there are known electrical heater devices
which comprise at least one metal electrode and a conductive polymer in
contact therewith, wherein the metal surface which contacts the conductive
polymer has a roughened or otherwise treated to improve its adhesion to
the conductive polymer. The metal electrode is preferably an
electrodeposited foil. The conductive polymer preferably exhibits PTC
behavior. The electrodes of these devices cover the entire area of the
heater and there is only one current direction, namely that leading the
shortest way through the PTC layer, from one metal foil to the other.
While the described devices are claimed to include self-limiting heaters,
their characteristics are substantially different from those obtained with
the invention.
One of the objects of the invention is to provide a self regulating heating
device, a property of which is its relative insensitivity to large
variations in voltage at or near the thermal control temperature.
The invention will be described primarily in terms of composite devices
wherein one component exhibits a positive temperature coefficient of
resistance (PTC) and the other component exhibits essentially zero
coefficient of resistance (ZTC) behavior.
A problem inherent in prior art devices depending solely on the variation
of resistivity with temperature, has been that certain performance
characteristics of the device were not always obtainable. For example, it
is most desirable to maintain a given power output within a narrow control
temperature range, and this temperature range does not always coincide
with the anomaly temperature where most polymers exhibit their T.sub.s
(switching temperature) and which is closely associated with the melting
point in the case of crystalline polymers.
A feature of the invention is therefore to establish the control
temperature of the device further removed from its crystalline melting
point, since experience has shown that the closer a PTC component operates
to its melting point, the less stable it is.
SUMMARY OF THE INVENTION
These features are obtained in accordance with the invention by making the
components of the layered structure such that at room temperature, the
resistance in the PTC layer between the ZTC layers is very much less than
the resistance in the combined ZTC layers, which in turn is very much less
than the resistance in the PTC layer between the electrodes, and where at
control temperature the resistance in the PTC layer between the parallel
ZTC layers is equal to the resistance in the parallel ZTC layers, the
geometry being such that at the control temperature where the resistances
of the two components are equal, the heat generated per time and unit area
(the watt densities) are also essentially equal.
Due to these relationships in the invention the PTC layer at room
temperature acts as a short circuit between the parallel ZTC layers. But
because of geometry the resistance between electrodes in the PTC layer is
very high when voltage is at first applied and the ZTC layers alone
develop heat. However as the temperature rises the resistivity in the PTC
layer increases until the resistance between the ZTC layers is equal to
that of the combined ZTC layers. Slightly above this temperature the two
ZTC layers act as electrodes and heat is generated uniformally throughout
the system, and any further rise in temperature anywhere in the area of
the ZTC layers effectively reduces or shuts off the current. In this way
the PTC component acts almost only as a control, and the ZTC components
perform as the active heating elements.
Under controlling conditions, because the ZTC layers act as electrodes,
heat is generated over the entire area,- even outside the current carrying
electrodes.
Because the initial current is independent of the resistance in the PTC
control layer, variations of as much as 2 to 5 times the room temperature
original PTC resistance play almost no role. Therefore the initial power
output is unchanged in spite of a considerable degree of instability in
the PTC layer.
Furthermore the cutoff, or control temperature is only slightly by large
variations in voltage. Since the heating function is carried out mainly by
the ZTC layers these elements have almost no inrush.
Because the ZTC layers are the main source of heat, and the PTC layer acts
as the control in a current direction normal to the ZTC layers, the
characteristic `hot line` effect of a pure PTC element is completely
eliminated and the element generates even heat over the entire area, and
the temperature is regulated almost regardless of heat loss variations.
Because the PTC component is equal in area to the parallel ZTC components
the maximum watt density in the PTC occurs when the resistances are equal,
and at any higher temperature the density decreases rapidly. In this way
the PTC component is never highly stressed which is conductive to a long
and stable life.
A novel feature of the invention is the limit placed on watt density at the
control temperature. In the prior art patents much is said about the
resistance and the resistivity of the two active components, but nowhere
is watt density at control temperature mentioned. That this was not
recognized as a factor can be shown from the examples of the mentioned
patents. These clearly illustrate that the only concern was to control the
effective T.sub.s of the elements and that no consideration was given (in
fact no recognition of the problem) to the critical effect of relative
watt densities.
Furthermore, the test method described in the prior art patents cannot be
used to evaluate all the examples shown, because it would give false
indication of the performance to be expected under selfheating heating
conditions. In the test method described, the heating element was
energized with a small power input, for measuring the variation in
resistance of the whole element, but the temperature was controlled by an
outside source which therefore was not sensitive to heat generated
separately in the two components. This was important because in many cases
the area of the two components differed. Therefore in a selfheating mode,
such an element with a relatively small PTC component, but having the same
resistance as the ZTC component at the control temperature would
experience overheating in the PTC layer, and this could easily result in
failure.
The whole intent of the heating elements described in U.S. Pat. No.
4,017,715 is to provide a means for exceeding (if only temporary) the
melting point of the PTC layer. On the other hand the construction of the
elements described in some of the other prior art patents place no limits
on the temperature experienced by the PTC layer due to watt density or
voltage gradient effects.
Much has been written about the so-called `hot-line` effect, where, due to
the positive coefficient of resistance of a PTC element, the tendency is
for heat to be unevenly generated along a line midway between the two
electrodes. This is especially the case where higher watt densities are
experienced, or especially where there is insufficient heat loss so that a
portion of the PTC film overheats. Another result of the hot line effect
is that the voltage gradient in the area of the hot line becomes very
great, and failure may actually occur first because of a dielectric
breakdown effect. At any rate, to promote stability, the voltage gradient
at control temperature, when the resistances are equal, must also be equal
and this is the result of equal geometry as well as 100% electrical and
thermal contact.
The PTC portion of a series PTC-ZTC element cannot shut off the power of
the whole element until its resistance equals and then exceeds the
resistance of the ZTC portion at that temperature. The temperature that
each component of the element attains is a function of the power density
inherent in its individual operation, and if the power density in the PTC
component is very high when shut down occurs, its local temperature can be
very high. Polymeric PTC materials are notably unstable close to or above
the melting point of the plastic, which in turn is associated with the
T.sub.s temperature.
Therefore, it is a critical factor in the invention to make series PTC-ZTC
elements where the PTC component performs its limiting effect at
temperatures well below the melting point of the plastic, or somewhat
below its T.sub.s temperature. To accomplish this it is therefore
essential that the heat generation or the watt density of the PTC
component must not exceed the watt density of the ZTC component at the
control temperature.
If the same terminology is used as in the mentioned prior art patents, U.S.
Pat. Nos. 4,177,376, 4,330,703, 4,543,474, and 4,654,511 are hereby
incorporated by reference then the PTC component of type 3 or 4 would be
preferred over the sharp cutoff depicted for types 1 and 2. With such a
PTC component, and assuming the cut off of the series element to occur
when R(PTC)=R(ZTC), then the cut off temperature may be regulated over a
wide range, and well below the melting point of the plastic or its type 1
T.sub.s. In fact, making use of non-crystalline polymers with sufficient
coefficient of expansion, but with no real melting point, would be more
desirable. Because of the inherent instability of the PTC compositions and
especially near the melting point (i.e. T.sub.s) of crystalline polymers
it is therefore important to develop limiting temperatures well below the
melting point of the polymer in PTC component, or to make use of a
suitable non crystalline polymer. This is in contrast to the
configurations developed by prior workers in the field where cutoff is
desired as close as possible to the Ts of the matrix polymer.
The present invention is thus directed to the relationship between the watt
densities at the cutoff temperature, i.e. where resistances of the two
components are equal. The same principle holds, for example, for a series
construction between parallel facing electrodes. Here, again, the PTC
control layer is protected from excess voltage gradients and watt
densities by the coextensive layers of the ZTC or NTC material.
BRIEF DESCRIPTION OF THE DRAWINGS
The above mentioned and other features and objects of the invention will
clearly appear from the following detailed description of embodiments of
the invention taken in conjunction with the drawings, where
FIG. 1 shows a layered PTC-ZTC structure,
FIG. 2 is a resistance-temperature curve,
FIG. 3 shows an embodiment of a heating element, and
FIG. 4 illustrates the performance of the element according to the
invention as compared to elements made according to the prior art
technique.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 schematically illustrates a structure having two ZTC layers 1 and 2
with a PTC layer 3 in between. The layers are in full contact with each
other. Electrodes 4 and 5 are diagonally arranged in the ZTC layers,
within the layers as shown or in contact with the layers as an
alternative. 2RZTC is the resistance in each of the ZTC layers so that the
resistance of both layers in parallel is RZTC. The resistance across the
PTC layer is RPTC1 and the resistance along the PTC layer between the
electrodes is RPTC2.
In FIG. 2 is illustrated a curve showing the relationship between the PTC
resistance RPTC1 and the ZTC resistance as a function of temperature. At
room temperature the resistance across the PTC in the electrode area must
be very small compared to the resistance in the ZTC layers. Its function
is to couple the parallel ZTC layers. At the same time the resistance in
the PTC layer between the electrodes RPTC2, because of the ratio of
thickness to width, must be substantially greater than the resistance in
the ZTC layers, so that heat is generated almost only in the ZTC layers.
At control temperature the resistivity in the PTC layer has risen so that
its resistance between the ZTC layers equals the resistance in the ZTC
layers themselves. Once the heating element has reached the control
temperature the wattage output remains virtually unaffected despite
substantial increases in voltage.
Another way of expressing the relationships between the resistances, and in
more detail, is as follows:
The relationship between RPTC1 and RPTC2 is inherently taken care of by the
geometry involving the ratio between the thickness of the PTC layer and
the distance between the electrodes. The main requirement is that the
resistances and thus power development at the control temperature are
equal. Again, because of the 100% contact area of the PTC and ZTC
components the watt density requirements are fulfilled.
The reason for the requirement that, at control temperature, the
resistances in the PTC and ZTC components are equal, is that below this
temperature, essentially all power is developed in the ZTC layers. When
the resistance in the PTC layer exceeds the resistance in the ZTC layer,
and because of the nature of the PTC resistance temperature curve, the
heat will be generated predominantly in the PTC layer, but this also means
that the resistance of the whole composite rapidly increases. In other
words, at temperatures below the control temperature the characteristics,
power output, stability etc are only a function of the ZTC component and
the PTC provides only the limiting control.
The calculation of the relative values of resistance and resistivity in a
given heating element can then go through the following procedure. In
essence this is the same as that outlined above.
The effects of geometry on effective resistance in the PTC layer are as
follows:
The resistance through the PTC layer is greatly dependant on the direction
of current flow. For example, the resistance through the thickness of the
PTC layer is very small compared to the resistance from electrode to
electrode through the width of the layer. And furthermore the resistance
across the PTC layer in the limited area of the electrodes must also be
small compared to the resistance in the parallel ZTC layer.
Since
RPTC2=ohmcm.multidot.d/(t.l)
and
RPTC1=ohmcm.multidot.t/(d.l)
then
RPTC1=RPTC2.multidot.t.sup.2 /d.sup.2
Depending on the ratio of distance between the electrodes (d) and the
thickness of the PTC layer (t), the resistance in the vertical direction
will at all times be t.sup.2 /d.sup.2 times that in the horizontal
direction.
Since at room temperature the ZTC resistance must be very much less than
the RPTC2 and very much greater than the RPTC1, this sets limits on these
resistance values in relation to the geometry of the device. But to be
fully effective, at room temperature, the resistance through the PTC layer
only in the area of the electrodes must also be so small compared to the
ZTC resistance, that it acts as a coupling short circuit between the two
ZTC layers, and then the watt density developed in this area is no greater
than the watt density developed in the combined ZTC layers. Under these
conditions the current will flow essentially straight across the PTC layer
at each electrode, and then through the ZTC layers to the opposing
electrodes. Now, as the temperature rises and the resistivity in the PTC
layer increases, more and more of the PTC layer conducts current between
the ZTC layers until a temperature is reached where the resistance offered
by the PTC layer equals the resistance of the ZTC layers. Under these
conditions equal heat is developed in both components. If the temperature
rises still further, the PTC component in the series starts to limit the
current passing through it and hence through the whole device. At this
control temperature the current is flowing in effect half through the PTC
and half through the ZTC and if the temperature increases further, due to
for example an increased voltage, the resistivity in the PTC component
continues to rise while the current flows more and more vertically through
the PTC and less and less horizontally through the ZTC. This then
surprisingly causes a compensating reduction in element resistance, which
has the effect of maintaining a controlled wattage output over the whole
control temperature range.
EXAMPLES
The relationship between the geometry of the heating elements and the PTC
and ZTC compositions used to make the elements will clearly appear from
the following examples, with reference to FIG. 3 where two ZTC layers 11
and 12 are separated by a PTC layer 13. Electrodes 14 and 15 are connected
diagonally to the ZTC layers. The PTC layer has a thickness t, a length l
and a distance d between the electrodes 14, 15 which is equal to the
length l, when the heater element is formed as a square.
In the squarely formed elements tested, the size of the elements varied
from d=1.6 cm to d=45 cm. The thickness of the PTC layer varied from 0.05
to 0.1 cm, and the thickness of the combined ZTC layers from 0.0032 to 0.1
cm.
It is a requirement of the invention that at cutoff or control temperature:
RPTC1=RZTC,
where RPTC1 is the electrical resistance measured across the PTC layer and
where RZTC is the resistance of the two ZTC layers connected in parallel,
each having a resistance of 2.RZTC.
Therefore, in an electrical square,
since
R=ohmcm.multidot.(distance between electrodes/area of electrodes),
then
RZTC=ohmcm(ZTC).multidot.d/(t(ZTC).d)=ohmcm(ZTC).multidot.1/t(ZTC)
and
RPTC1=ohmcm(PTC).multidot.t(PTC)/d.multidot.d
so that the ohmcm ratio
ohmcm(PTC)/ohmcm(ZTC)=d.sup.2 /(t(ZTC).multidot.t(PTC))
where
t(ZTC)=the combined thickness of the two ZTC layers, and
t(PTC)=the thickness of the PTC layer.
When the thickness of the PTC layer is 0.025 cm and the thickness of the
combined ZTC layers are 0.0032 cm (using glass scrim impregnated layers),
the ohmcm ratio values for the heaters are as follows, at control
temperature (CT) and at room temperature (RT):
______________________________________
d CT RT
______________________________________
1.6 32,000 3,200
4.5 250,000 25,000
45 2.5 .multidot. 10.sup.7
2.5 .multidot. 10.sup.6
______________________________________
The corresponding ratios for heaters made with extruded ZTC layers of 0.025
cm thickness were:
______________________________________
d CT RT
______________________________________
1.6 2,048 204
4.5 16,200 1,620
45 1.6 .multidot. 10.sup.6
1.6 .multidot. 10.sup.5
______________________________________
A comparison with a prior art heating element (Beispiel 5) shows an ohmcm
ratio of 20/7=2.85 at room temperature with a 2.54 times 2.54 size heating
element, whereas one of the heating elements of the same size have an
temperature. These ratios show the main difference between the prior art
concepts and the concept of this invention.
An example of a heating device made in accordance with the invention and
compared with a heating device made in accordance with the prior art as
represented by DE Patent No. DE-02-2 543 314 and U.S. Pat. No. 4,017,715
are as follows:
The PTC layer consisted of 45 parts of ELETEX carbon in 100 parts of PE
(polyethylene) or EVA (ethylene vinyl acetate) resin. The compound was
made into a 0.1 cm thick film at a resistivity of 4.multidot.10.sup.4
ohmcm at room temperature. The ZTC layers consisted of a glass scrim of an
open structured glass paper, mat or cloth impregnated with an aqueous
dispersion of KETCHEN BLACK. The Ketchen Black was run through a fluid
energy machine along with 20% by weight of 40% colloidal silica (Dupont
LUDOX HS-40). The fluid energy machine was utilized to modify the surfaces
of the carbon particles by impinging the particles against one another in
a high velocity gas stream. This material was dispersed in water along
with 5% polyethyleneimine (PEI) to effectively wet the carbon black and
control the charge on the carbon particles. The coating is modified with a
binder consisting of an acrylic latex, clay and colloidal silica and also
PEI, the binder being in a proportion to produce the desired resistance
level on the coated scrim.
Thus with the PTC layer at a resistivity of 4.multidot.10.sup.4 ohmcm and
an ohm/square resistance of 400,000 combined with two ZTC layers of 3,000
ohm/square each (together in parallel 1,500 ohm/square) this device had at
room temperature a resistance of 1,520 ohm where the area between the
electrodes was 6.3.multidot.6.3 cm.
For comparison a sample was made according to the principles outlined in
the prior art patents, the ohm/square resistance in the PTC layer was
1,600 and the ohm/square resistance in the combined ZTC layers was 15,000
ohms.
As will be seen from the above, the resistances of the components of the
prior art device at room temperature, have the following characteristics:
RPTC1<<RPTC2<<RZTC,
which is quite different from the resistance relationships of the
components according to the invention.
The room temperature resistance of the two samples, before connecting them
to a power supply, was comparable, but the performance, when connected to
a power supply, was very different as can be seen from the curves
illustrated in FIG. 4, especially with respect to the increasing voltage
in the control temperature region.
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